1. Field of the Invention
The present invention relates to a friction plate for a wet multi-plate clutches for use in a clutch or a brake in an automatic transmission.
2. Description of Related Art
FIG. 1 is a sectional view showing a basic configuration of a wet multi-plate clutch 10, in which power is transmitted between a clutch casing 21 and a hub 22 via separator plates 30 and friction plates 40. Reference numeral 23 denotes a spline groove to which the separator plates 30 fit, 24 a spline groove to which the friction plates 40 fit, 25 a piston for pushing the separator plates 30 and the friction plates 40 against a backing plate 26, 27 a snap ring which supports the backing plate 26, and 28 a seal ring.
In recent years, there are more and more demands for further improvement in fuel economy of motor vehicles, and as a part of the improvement in fuel economy, there are also increasing demands for further reduction in dragging torque generated between friction plates and separator plates at idling in automatic transmissions of motor vehicles.
To cope with the demands, there have been considered friction plates which include oil grooves which are closed at end faces in order to facilitate the separation of friction plates and separator plates when a clutch is idle and oil grooves which pass across the friction plate in radially inward and outward directions for supply oil for preventing seizing when the clutch is engaged.
In recent years, in order to improve in gear shift response which is aimed at improving engine performance as well as fuel economy, clearances between friction plates and separator plates are getting narrower than before, and hence, dragging torque between the friction plates and the separator plates tends to increase due to oil film left interposed therebetween when the clutch is idle.
In conventional friction plates, the discharge of oil from friction surfaces was not sufficiently performed, and therefore, the demand for further reduction of dragging torque could not be satisfied. In particular, in a low rotational speed region, the discharge of oil left interposed between the friction plates and the separator plates could not be implemented sufficiently, and therefore, the dragging torque could not be decreased.